1. Field of Invention
The present invention relates to a voltage regulator, in particular to a voltage regular capable of providing an indicator signal to a load, and a corresponding method.
2. Description of Related Art
It is sometimes required for a voltage regulator to provide an indicator signal to its load. For example, when the load is a certain kind of central processor unit (CPU), the voltage regulator may need to provide power information so that the CPU can operate under the best current-voltage relationship, and adjust its operational frequency accordingly.
More specifically, referring to FIG. 1, it is required in the specification of some CPU that the supply voltage must decrease in correspondence with the increase of the load current. The corresponding current-voltage relationship is shown in FIG. 2, wherein the solid line represents the desired relationship with a slope RLL (decided by the specification), and the dash lines represents the tolerable error range.
To meet this requirement, the voltage regulator needs to obtain current information from outside of the circuit, to feedback such information into the circuit to control its output voltage. Referring to FIG. 3, the voltage regulator 10 includes an error amplifier 12, a voltage regulator module (VRM) 14, and driver gates 16 and 18. The output voltage Vout (or a dividend voltage thereof), after voltage drop, is compared with a reference voltage Vref, and the result is input to the VRM 14 to generate up-gate and low-gate control signals, which control the operations of the up-gate and low-gate transistors Q1 and Q2, respectively. In addition, the VRM 14 further obtains load current information from outside of the circuit, typically by sensing the current flowing through the inductor L. Thus, the VRM 14 controls the output voltage Vout according to the load current information, the current-voltage relationship in FIG. 2, and the comparison made by the error amplifier 12, so that a desired output voltage Vout is generated. The resistor Rdcr represents the parasitic resistance of the inductor L, and a typical way to sense the current on the inductor L is to detect the voltage across this resistor Rdcr; the detected voltage signal can be used to represent the load current information. FIG. 4 shows a typical method to detect the voltage across the resistor Rdcr. A resistor and a capacitor connected in series are connected with the inductor L in parallel; the voltage across the capacitor is converted to current by a transconductor 15, which is the desired load current information.
However, because the parasitic resistor Rdcr of the inductor L has positive temperature coefficient (PTC), the load current information generated according to the voltage across the resistor Rdcr will be inaccurate outside a certain temperature range. Hence, as shown in FIG. 3, the circuit must somewhere include a resistor having negative temperature coefficient (NTC) to compensate the temperature effect of the load current information to the VRM 14, which may be the resistor Rntc1 located in the position as shown, so that the output of the error amplifier 12 carries negative temperature coefficient, and the total effect is compensated in the VRM 14.
As stated above, in addition to generating the desired output voltage Vout, the voltage regulator 10 also has to provide power information (the product of load current and output voltage). However, the current information obtained from the voltage across the resistor Rdcr is inaccurate, so an NTC resistor Rntc2 is required to compensate the temperature effect on the current information, as shown in FIG. 4. The current information after compensating temperature effect can then be used to calculate power.
The above mentioned prior art is disadvantageous in that additional NTC resistors in such prior art increase the cost (even higher cost for PTC resistors). Moreover, if the load requires a multiple phase voltage regulator, undesirably, corresponding number of additional NTC (or PTC) resistors are required.
In view of the foregoing drawbacks in the prior art, it is desired to provide a voltage regulator capable of providing an indicator signal to its load, without using costly devices.